This invention relates to an improved process for deoxidation and desulfurization of steels. One of the natural impurities in steel is sulfur. Too much sulfur tends to make steel "hot-short", i.e., during rolling surface cracks develop as a result of iron-sulfide inclusions. The effect of iron sulfide inclusions is often minimized by the addition of manganese to the steel to form manganese sulfide inclusions which do not have the same damaging effect. Recently, however, the use of calcium metal additions to the melt has been pursued as a means of substantially eliminating the sulfur from the steel. Lower sulfur steels are becoming increasingly important. Consider, for example, that steel parts for the Alaskan Pipe Line were fabricated substantially entirely from low sulfur steels.
U.S. Pat. No. 3,575,695 teaches desulfurization of molten steel by adding calcium and calcium alloys to the lower portion of a molten bath which has been subjected to a preliminary deoxidation step. The preliminary deoxidation step involves the typical manganese, silicon or aluminum additions to the batch. Because of its relatively high vapor pressure at steelmaking temperatures, calcium metal (as a powder, granule, or small lump) is added to the bath through a tube extending thereinto. The calcium metal is carried into a bath by an inert gas such as argon. U.S. Pat. Nos. 3,885,957 and 3,980,469 describe very similar processes for desulfurizing steel in a ladle by blowing calcium or calcium containing substances into the ladle through a refractory tube. According to the '957 patent only about ten percent of the calcium added is converted to calcium sulfide. In other words, the process is only ten percent efficient at reacting calcium with the sulfur in the metal to thereby remove the sulfur from the metal. This very low utilization rate of calcium metal is only one disadvantage of the blowing process for introducing calcium or calcium alloys. The large amounts of argon needed for the blowing process are expensive and create a pollution problem as the argon rises out of the molten metal bath carrying dust with it. Further, the refractory tube for introducing the argon and calcium is a continuing maintenance problem.
Because of the low utilization efficiency of the calcium metal using the blowing technique, the steel is first treated with expensive vacuum degassers and with aluminum additions to remove excess oxygen prior to the calcium addition. This allows the injected calcium or calcium alloy to combine directly with the sulfur in the steel without being diverted to deoxidizing the melt. (Calcium is more reactive with oxygen in the melt than with sulfur in the melt).
It is an advantage according to this invention to provide an improved method of deoxidizing and/or desulfurizing molten metal, for example, iron, steel and copper melts.